* Kim Jong Un and Moon Jae-in meet for third time
* Moon lands in Pyongyang to discuss denuclearisation, end of war
* Agreeing timeline to denuclearisation key after Kim pledge
* N.Korea says U.S. is to blame for stalled nuclear talks (Adds Kim and Moon's arrival, schedule of official talks)
By Hyonhee Shin and Joyce Lee
SEOUL, Sept 18 (Reuters) - North Korean's Kim Jong Un greeted Moon Jae-in with hugs and smiles on Tuesday as the South Korean president arrived in Pyongyang to discuss faltering talks on denuclearisation and the prospect of officially ending the Korean War.
Hundreds of North Koreans wearing suits and traditional dresses also greeted Moon, carrying flowers and waving Korean peninsula and North Korean flags. A sign behind them read: "We ardently welcome President Moon Jae-in's visit to Pyongyang!"
The two leaders stepped out of the same black Mercedes vehicle with open-top rear seats to arrive at Paekhwawon State Guest House, where Moon will stay.
The guesthouse was also used by two former South Korean presidents during their own summits with Kim's father, Kim Jong Il, in 2000 and 2007.
Kim and Moon will hold formal talks from 3:30 pm to 5 pm (0630 to 0800 GMT), Moon's office said.
The inter-Korean summit, the third between Moon and Kim, will be a litmus test for another meeting Kim has recently proposed to U.S. President Donald Trump.
Trump has asked Moon to be "chief negotiator" between himself and Kim, according to Moon's aides, after Trump cancelled a trip to Pyongyang by his secretary of state last month.
Washington wants to see concrete action toward denuclearisation by North Korea before agreeing to a key goal of Pyongyang - declaring an end to the 1950-53 Korean War.
"If North Korea-U.S. dialogue is restarted after this visit, it would have much significance in itself," Moon said before his departure.
Underscoring the challenges ahead, North Korea's official Rodong Sinmun said on Tuesday "the responsibility falls squarely on the United States" for the stalled nuclear discussions.
"It is due to its nonsensical, irrational stubbornness that other issues can only be discussed after our country has completely verifiably, irreversibly dismantled our nuclear capabilities... without showing the intention to build trust including declaring the end of war," the newspaper said in an editorial.
Moon, himself the offspring of a family displaced by the war, has met Kim twice this year at the border village of Panmunjom.
As he landed at Pyongyang's Sunan International Airport on Tuesday morning, Moon was greeted by Kim, his wife Ri Sol Ju and other top North Korean officials, as well as a large honour guard and a military band.
Kim Yo Jong, the younger sister of the leader and a key propaganda official, was seen preparing officials for Moon's arrival and accompanying Kim Jong Un and his wife.
South Korean corporate executives, including Samsung Electronics Vice Chairman Jay Y. Lee and the chiefs of SK Group and LG Group, will meet with North Korean Deputy Prime Minister Ri Ryong Nam, who is in charge of economic affairs.
On Wednesday, Moon and Kim plan to hold a second round of officials talks after which they are expected to unveil a joint statement, and a separate military pact designed to defuse tensions and prevent armed clashes. Moon will return home early Thursday.
This week's summit comes as the United States presses other countries to strictly observe U.N. sanctions aimed at choking off funding for Pyongyang's nuclear and ballistic missile programmes.
North Korea says it has destroyed its main nuclear and missile engine test site, and has halted atomic and ballistic missile tests but U.S. officials and analysts believe it is continuing to work on its weapons plans covertly.
U.S. Ambassador to the United Nations Nikki Haley accused Russia on Monday of "cheating" on U.N. sanctions on North Korea.
Moon is hoping to engineer a proposal that combines a framework for the North's denuclearisation and a joint declaration ending the Korean War, Seoul officials said.
The conflict ended with an armistice, not a peace treaty, leaving U.S.-led U.N. forces including South Korea technically still at war with the North.
But U.S. officials remain "unenthusiastic" about declaring an end to the war without any substantial action toward denuclearisation from the North, Seoul officials said.
South Korea is pinning high hopes on Kim's remarks to Moon's special envoys earlier this month that he wants to achieve denuclearisation within Trump's first term in office ending in early 2021.
Agreeing on a timetable is a core task for Moon, as it would induce U.S. action, said Lee Jung-chul, a professor at Soongsil University in Seoul.
"Given U.S. scepticism that South Korea may have oversold Kim's willingness to denuclearise, how President Moon delivers his sincerity toward denuclearisation to Trump would be a key factor for the fate of their second summit," Lee told a forum on Monday in Seoul. (Reporting by Hyonhee Shin, Joyce Lee, Soyoung Kim and Pyongyang Press Corps. Editing by Lincoln Feast.)